The Centauri never really got around to us. They've been in a
decline for a long time; the Narn occupation was one of the last of their
imperialist rampages. Now they're pulling back further and further. And
also as re: Earth...space is big. 100 years ago, we weren't putting out
any radio or television or microwave transmissions; they can't check EVERY
planet. We got overlooked for a long time by lots of different groups.

Re: weapons...the Narns got their hands on the Centauri leftover
weapons, and the manufacturing systems put in place there during the
occupation that nearly strip-mined their world. Using that as a base,
they got heavily into the weapons business, and continue to do so.

jms